If you have a stack of 10 units under 1 general and another 10 units in another stack with a general. They will all be under your command. the only difference is you can deploy your 1 general whereas the other general willl come on as reenforcements and will be a bit aways from the main battle for a while.

But if 1 of those stacks has 11 units in it with or without a general it will be controlled by the A.I.